Vehicular powertrains commonly include conventional multi-ratio, automatic transmissions and electro-hydraulic controls for effecting speed ratio changes or gear shifts. Controllable torque transfer devices, commonly clutch packs or band clutches, are utilized to transfer torques between planetary gearset members to effect speed ratio changes. Each such shift typically comprises at least one oncoming clutch application or offgoing clutch release. Often times, at least one offgoing clutch release and one oncoming clutch application is associated with a speed ratio change.
Clutch apply and release timings are critical to effective speed ratio shift control. Accurate knowledge of clutch apply or release state can significantly affect shift quality. Inaccuracy in clutch state knowledge can result in excessive and premature clutch wear. Additionally, loss of clutch state knowledge can result in significantly reduced vehicle performance since substantial compromises in shift controls may need to be made absent such knowledge. And, loss of clutch state knowledge may even result in vehicle disablement.
Typically, clutch states are determined from direct sensor inputs. For example, a hydraulically actuated clutch system may include pressure switches or transducers in communication with respective clutch control chambers for inferring clutch application when pressure is sensed above a predetermined threshold and clutch release when pressure is sensed below a predetermined threshold. Loss of pressure signals from such sensors can occur from a variety of factors including, for example, sensor failure, connector or wiring faults, and even internal controller faults.
Electrically variable transmissions are know which also utilize electro-hydraulic controls to effect clutch applications and releases. Clutch applications and releases in such transmissions are somewhat similarly utilized to establish ranges of speed ratios corresponding to selected gearsets in accordance with certain clutch application and release combinations. The number of degrees of freedom in an electrically variable transmission can make clutch state knowledge even more critical to operation than in a conventional multi-ratio, automatic transmission.
Therefore, what is needed is a robust system for determining clutch states in a electro-hydraulically controlled transmission. Particularly, it is desirable to provide for a robust diagnostic and recover control suitable for application to an electrically variable transmission.